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When dealing with segmentation in x86, it is *usually* illegal to attempt to access a segment which has a null selector when in protected mode and not in 64 bit mode. While this is *almost* true, it is not actually technically true. What actually *is* true is that if you *set up* a segment using a null selector in those circumstances, that segment becomes unusable, and then tryint to use it causes a fault. When in real mode, it is perfectly legal to use a null selector to access memory, since that is just a selector with numerical value 0. When you then transition into protected mode, the selector would still be 0 (a null selector), but the segment itself would still be set up properly and usuable using the base value, limit, and other attributes it carried over from real mode. Rather than check if a segment has a null selector while handling segmentation, it's more correct for us to keep track of whether the segment is currently usable and check that in the TLB. Change-Id: Ic2c09e1cfa05afcb03900213b72733545c8f0f4c Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/55245 Maintainer: Gabe Black <gabe.black@gmail.com> Tested-by: kokoro <noreply+kokoro@google.com> Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
This is the gem5 simulator. The main website can be found at http://www.gem5.org A good starting point is http://www.gem5.org/about, and for more information about building the simulator and getting started please see http://www.gem5.org/documentation and http://www.gem5.org/documentation/learning_gem5/introduction. To build gem5, you will need the following software: g++ or clang, Python (gem5 links in the Python interpreter), SCons, zlib, m4, and lastly protobuf if you want trace capture and playback support. Please see http://www.gem5.org/documentation/general_docs/building for more details concerning the minimum versions of these tools. Once you have all dependencies resolved, type 'scons build/<CONFIG>/gem5.opt' where CONFIG is one of the options in build_opts like ARM, NULL, MIPS, POWER, SPARC, X86, Garnet_standalone, etc. This will build an optimized version of the gem5 binary (gem5.opt) with the the specified configuration. See http://www.gem5.org/documentation/general_docs/building for more details and options. The main source tree includes these subdirectories: - build_opts: pre-made default configurations for gem5 - build_tools: tools used internally by gem5's build process. - configs: example simulation configuration scripts - ext: less-common external packages needed to build gem5 - include: include files for use in other programs - site_scons: modular components of the build system - src: source code of the gem5 simulator - system: source for some optional system software for simulated systems - tests: regression tests - util: useful utility programs and files To run full-system simulations, you may need compiled system firmware, kernel binaries and one or more disk images, depending on gem5's configuration and what type of workload you're trying to run. Many of those resources can be downloaded from http://resources.gem5.org, and/or from the git repository here: https://gem5.googlesource.com/public/gem5-resources/ If you have questions, please send mail to gem5-users@gem5.org Enjoy using gem5 and please share your modifications and extensions.